1. Field of the Invention
This invention relates to an electronic circuit for the protection of switches, such as reed switches or other electrical contacts, from electrical transients and surges. Transient electrical conditions occurring during electrical contact opening and closing can have adverse effect on switch life and the circuitry of this invention reduces such adverse effects.
2. Description of Related Art
Electrical transients and surges seriously shorten the operating life of switch devices such as reed switches. These voltage and current surges and transients typically show up when the switch is used to directly control high current, inductive, or capacitive loads. Such loads typically include relays, air valve control solenoids, inductive motors, long cable runs, bypass capacitors, and large wattage incandescent light bulbs. In addition, transient loads can be induced on the power line by lightning storms and the operation of high power or high power electrical machinery.
Switch operation, and in particular reed switch operation, is old and well-known, and manufacturers design equipment with high concern for reliability and with long life as a consideration. Typically, manufacturers suggest a hundred million plus operations as life cycle tests. Such tests, however, are conducted generally with low current loads, such as 10 mA and pure resistive loads in the external circuits. The result is that, while such reed switches can endure this low current and passive loads in cycle tests, when such devices are operated at their rated capacity, such as 1 amp and 200 volt maximum, the expected life of the reed switch is very significantly reduced. Electrical contacts on such switches can deteriorate much more rapidly than their normal life expectancy, resulting in the contacts pitting, degrading, and welding together. Such switch units often fail by having their contacts stick or weld together, or by creating a high resistance electrical contact surface. When such failures occur in industrial applications the equipment is removed from service and inspected. Often when the device is ready for test the natural vibration and mechanical shock of removal from service will break the contact weld and provide a temporary "fix" to the switch contact problem. This results in the equipment being returned to industrial service and providing a machine whose operation is intermittent and unpredictable. When operating in an unprotected mode, such as a fluid cylinder proximity circuit, reed switches can fail after the first 5,000 to 50,000 operations. This is a highly undesirable reduction from the desired life of such switch.
Prior solutions to these problems have included the use of a diode across the inductive load. While this method works well in some applications, it will not work for AC applications. In addition the person installing the diode must be aware of the circuit operation and the polarity of the diode relative to the solenoid and the circuit voltages.
Other solutions have been to use diodes in combination with zener diodes or other semiconductor devices to absorb spikes. In addition, RC networks and magnetic oxide varistors in parallel with the inductive circuit load have also been tried. Use of RC networks require that the resistance and capacitance values be selected according to the characteristics of the inductive load, and therefore must be selected and coordinated for each specific application.
Disadvantages of the above solutions to the problem include the difficulty of containing proper polarity of protection circuits, the limitations of many circuits to only DC operation, and the minimal improvement in life expectancy of switch operation provided by any of the circuits.